Air vs. Surface Sampling Techniques
Mold tests can be divided into two categories: air and surface. If you find it necessary to perform a mold test (e.g. for legal purposes), then it would be advisable to take at least one surface sample and one air sample.
The reason is, in some situations, you may have mold growing on surface, yet it has not reached a point where it is releasing very many mold spores into the air. In which case, air sampling alone would provide results that did not accurately portray the extent of mold growth.
Or, you may have a situation where, in your random surface sampling, you did not sample a surface where mold was growing (or at least not at a significant level), but mold colonies throughout other parts of the home had reached a point where they were releasing substantial amounts of mold spores into the air. In this case, your surface sampling would not give you and accurate picture of the problem.